This application is entitled to the benefit of and incorporates by reference essential subject matter enclosed in Japanese Patent Application No. 2001-301798 filed Sep. 28, 2001.
1. Field of the Invention
The present invention relates to a control device of common rail fuel injection system of an engine chiefly adapted for a diesel engine and in particular relates to a method of controlling an electromagnetic valve that adjusts the quantity or rate of supply of fuel to a high-pressure pump.
2. Description of the Related Art
In a common rail type fuel injection system of an engine, in particular, a diesel engine, high-pressure fuel raised to an injection pressure (from a few tens to a few hundreds of MPa) is accumulated on a common rail and this fuel is injected into the cylinder by opening of an injector valve. Fuel supply to the common rail is effected by intake and discharge of fuel under approximately normal pressure accumulated in a fuel tank by means of a feed pump, followed by supply under pressure of fuel discharged therefrom to the common rail after being pressurized by a high-pressure pump.
An electromagnetic valve whose degree of opening is controlled in accordance with a duty signal is provided between the feed pump and the high-pressure pump and the quantity of supply of fuel to the high-pressure pump is controlled by this electromagnetic valve. That is, when it is desired to raise the common rail pressure comparatively abruptly, the degree of opening of the electromagnetic valve is made large, so that a comparatively large amount of fuel can be supplied to the high-pressure pump. As a result, a comparatively large amount of fuel is supplied under pressure to the common rail by the high-pressure pump, causing the common rail pressure to rise comparatively abruptly. Contrariwise, when it is desired to raise the common rail pressure comparatively slowly, the degree of opening of the electromagnetic valve is made small, so that a comparatively small amount of fuel is supplied to the high-pressure pump. In this way, a comparatively small amount of fuel is supplied under pressure to the high-pressure pump, so the common rail pressure is raised comparatively slowly.
A duty pulse of a prescribed duty ratio is supplied to the electromagnetic solenoid of the electromagnetic valve and the degree of opening of the electromagnetic valve is controlled in accordance with the duty ratio. The duty ratio and the degree of opening of the electromagnetic valve are continuously variable. Microscopically, as shown in FIG. 4, an ON/OFF signal as shown in (a) is repeatedly supplied to the electromagnetic solenoid of the electromagnetic valve. This causes a current of sawtooth shaped waveform as shown in (b) to flow in the electromagnetic solenoid, with the result that the valve body is actuated in response to this current. The mean current value Im changes in accordance with the duty ratio (in this case, the ratio of the ON time tON to the period Th), causing the valve body to be positioned essentially in a position corresponding to this average current value Im and resulting in minute vibrations accompanying the oscillation of the current, referred to this position. A drive current that produces such minute vibrations of the valve body is termed a dither current.
However, there are the following problems with a control device of common rail fuel injection system for a diesel engine mounted in a vehicle. Specifically, in the ordinary running condition etc, the operating state of the vehicle and the engine is continually changing and the target common rail pressure also changes in response thereto. Consequently, the quantity of fuel supply to the high-pressure pump i.e. the degree of opening of the electromagnetic valve, also changes in accordance with the changes of target common rail pressure. The control frequency in duty control of the electromagnetic valve is therefore set to an optimum frequency that is comparatively high so as to be able to track such changes of operating condition.
Conventionally, however, this control frequency was fixed irrespective of the operating condition of the engine and the vehicle. The so-called stick/slip problem was therefore produced of the valve body of the electromagnetic valve becoming stuck if the operating condition was fixed. Specifically, whereas the control frequency was set to a comparatively high frequency taking into account tracking characteristics etc of the operation of the electromagnetic valve in the high-speed range, if this was done, sticking of the valve body occurred during idling (when the valve body of the electromagnetic valve is fixed with a slight degree of opening) and/or during non-injection condition (fuel cut-off), as in the case of engine braking (when the valve body of the electromagnetic valve is fixed in the fully closed position). This is because, while frictional force due to viscosity of the fuel and/or friction constantly acts on the valve body sliding section, as shown in FIG. 4(b), at high frequencies the energy per current wave or the amplitude Ih are comparatively small, and are therefore insufficient to create minute vibrations of the valve body.
When such sticking of the valve body occurs, in order to subsequently move the valve body, it must be driven to overcome the force of static friction. Controllability is therefore adversely affected in that the valve body cannot perform tracking movement in the event of change of operating condition from fixed operating condition.
Also, in recent years, desulfurization of fuel (light oil) is carried out as a counter-measure against particulate material (PM); if this is done, the coefficient of friction of the fuel may become about twice that which is obtained conventionally, so the probability of sticking is further increased.
The present invention was made in view of the above problems, its object being to provide a control device of common rail fuel injection system of an engine wherein sticking of the valve body of the electromagnetic valve under idling conditions or under non-injection conditions can be prevented.
According to the present invention there is provided a control device of common rail fuel injection system of an engine comprising: a high-pressure pump that pressurizes to a high pressure fuel supplied from a feed pump; an electromagnetic valve for adjusting the quantity of fuel supply to the high-pressure pump located on a passage between this feed pump and high-pressure pump; and electromagnetic valve control means that controls the degree of opening of this electromagnetic valve in accordance with a duty signal, further comprising: detection means that detects an operating condition of the control device such that the degree of opening of said electromagnetic valve is constant and control frequency alteration means that alters the control frequency of said duty signal to a lower frequency when such an operating condition is detected.
With the present invention, since the control frequency of the duty signal is altered to lower frequency under a condition such that the degree of opening of the electromagnetic valve is constant, the energy or amplitude per current wave flowing in the electromagnetic solenoid can be made larger, thereby enabling minute vibrations of the valve body to be produced. In this way the valve body can be prevented from sticking.
The operating condition in which the degree of electromagnetic valve opening is constant may be idling condition or non-injection condition of the engine.
The control device may be for the engine for a vehicle and the detection means may identify the operating condition that the degree of opening of said electromagnetic valve is constant when the engine speed is the idling speed, the transmission is in neutral position and the accelerator pedal is in the fully closed position.
Also the control device may be for the engine for a vehicle and the detection means may identify the operating condition that the degree of opening of said electromagnetic valve is constant when the engine speed is higher than the idling speed, the transmission is shifted in any gear position and the target fuel-injection quantity to the engine is zero.
Also, when the control frequency of the duty signal is altered to lower frequency, the control frequency alteration means may correct the duty ratio of the duty signal such that the average value of the current flowing in the electromagnetic solenoid of the electromagnetic valve is the same as if the control frequency were not altered.
Also the electromagnetic valve may comprise an electromagnetic solenoid that is supplied with the duty signal, a spool-shaped valve body that is actuated in response to current flowing in this electromagnetic solenoid and a spring that biases this valve body in the opening direction.
Also the control frequency of said duty signal may be normally at least 170 Hz and less than 190 Hz but at least 120 Hz and less than 170 Hz when altered by the control frequency alteration means.
Also according to the present invention an electromagnetic valve control method for a control device of common rail fuel injection system of an engine wherein fuel supplied from a feed pump is pressurized to high pressure by a high-pressure pump and the quantity of fuel supply to this high pressure pump is adjusted by an electromagnetic valve whose degree of opening is controlled in accordance with a duty signal comprises a step of detecting an operating condition in which the degree of opening of said electromagnetic valve is constant and a step of, when such an operating condition is detected, altering the control frequency of said duty signal to a lower frequency.
Also according to the present invention an electromagnetic valve control device for a fluid circuit comprising an electromagnetic valve for adjusting the amount of working fluid and electromagnetic valve control means that controls the degree of opening of this electromagnetic valve in accordance with a duty signal, comprises: detection means that detects an operating condition in which the degree of opening of said electromagnetic valve is constant and control frequency alteration means that alters the control frequency of said duty signal to a lower frequency when such an operating condition is detected.